A known reluctance type motor comprises a stator having a plurality of magnetic poles and a rotatably supported rotor having a plurality of salient-poles. A magnetic attraction force is generated between the stator magnetic poles being successively excited and their corresponding rotor salient-poles so as to rotate the rotor.
The reluctance motor is advantageous in that it generates a large output torque and requires no magnetic rotor. On the other hand, a conventional reluctance type motor has a disadvantage such that its application field is unexpectedly limited due to the difficulty of a high-speed operation.
That is, an exciting coil of the reluctance type motor has a large inductance, and thus the magnetic energy stored in the exciting coil becomes very large. Accordingly, it requires a significant time for storing or discharging the magnetic energy. In other words, a building-up and a trailing-edge of the exciting current are undesirably delayed.
Furthermore, the magnetic energy storage and discharge in a magnetic pole are so frequently repeated during one complete revolution that a torque reduction occurs together with a counter torque. Moreover, in a last stage of a current supply period, an exciting current not contributing to the generation of an output torque will increase to cause a large joule loss. As a result, an operational efficiency of the motor is lowered and, therefore, its rotational speed is remarkably reduced.
If so-called advanced-phase current supply method, in which an exciting current is supplied to the exciting coil well before the salient-poles enter the magnetic poles, is applied to drive a reluctance type motor at a high speed, a disadvantage is usually recognized such that the motor output torque will not be generated and a copper loss will take place in a section, for example, of a 30-degree advanced phase angle, or an output torque generating section becomes fairly short. More specifically, the motor output torque is reduced and a torque ripple will be generated.
If the number of salient-poles and the number of magnetic poles are increased in order to increase the output torque of the motor, the time periods required for building up and trailing off the exciting current will increase due to the stored magnetic energy, causing a marked fall of rotational speed; the construction of the motor becomes complicated; and its size inevitably increases. Especially, in the case of a three-phase full-wave reluctance motor requiring a greater number of salient-poles and magnetic poles, it is normally difficult to realize a high-speed rotation and a substantial reduction of its dimensions.
Moreover, if a high-voltage electric power source is used in order to sharply build up the exciting current at an initial stage of current supply to the exciting coil, other problems occur, such as the exciting current building up too steeply when the magnetic poles reach their magnetically saturated point to causing yibrations and electric noises.
Furthermore, another problem of the conventional reluctance type motor is that a large magnetic attraction force caused between the magnetic poles and the salient-poles acts in a wrong direction in which the large magnetic attraction force cannot contribute to an output torque generation. Also, there is another problem that an extremely large torque is generated immediately after the salient poles begin to face the magnetic poles, whereas the torque becomes small shortly before the salient poles directly face the magnetic poles, thereby causing ripple of the output torque.